The present invention relates to a wire dot printing head for an impact printer, and more particularly to a wire dot printing head for printing by driving a printing wire secured to the tip of an armature of the impact printer.
An impact printer executes printing on a printing medium by driving a printing wire, which presses an ink ribbon against the printing medium. Since printing media are readily available and comparatively cheap a well, this impact printer is widely used for various kinds of output devices, such as those of data processing systems.
Impact printers as described above can be classified into several categories, including the plunger type, the spring charge type and the clapper type.
The spring charge type of impact printer employs an armature which is supported by a bias leaf spring. A printing wire is secured to the armature. The armature is attracted in advance to a core by a permanent magnet against the elastic force of the above-described bias leaf spring. To initiate a printing stroke, a coil wound around the core is excited to generate magnetic flux in an opposite direction to that of the permanent magnet, whereby the armature is released. To achieve rapid printing, highly responsive wire dot printing heads of the spring charge type have been widely utilized.
FIG. 1 is a cross-sectional view illustrating a conventional spring charge type wire dot printing head.
In this drawing, a base 3, a ring 4, a permanent magnet 5, a magnet yoke 6, a spacer 7, a leaf spring 8 and an armature 9 are stacked in this order between a guide frame 1 and a cap 2. Reference number 10 designates a clamp spring.
On a resilient portion of the leaf spring 8, an armature 11 is disposed, and spring wire 12 is secured to the tip of the armature 11. The tip of the printing wire 12 is guided by a wire guide 13 so as to be directed toward a platen.
A core 14 is disposed toward the center of the base 3, and a coil 15 is wound around core 14. This coil 15 is fixed to a printed circuit board 17 through a coil bobbin 16. The coil 15 is connected electrically to the printed circuit board 17 through coil terminals 18. Between the printed circuit board 17 and the base 3, an insulation plate is inserted.
Further, numeral 20 denotes wire felt disposed in the wire guide 13, through which the printing wire 12 passes.
The above-described construction provides a magnetic circuit, and the magnetic flux generated by the permanent magnet 5 returns to the permanent magnet 5 after passing through the magnet yoke 6, the spacer 7, the armature yoke 9, the armature 11, the core 14, the base 3 and the ring 4. By this magnetic circuit, the armature 11 is attracted to the core and displaced. This displacement of the armature 11 accumulates distortion energy in the leaf spring 8 so that the leaf spring 8 is put in a biased condition.
Under this biased condition, when a magnetic flux having a direction opposite to that of the magnetic circuit is generated by exciting the coil 15, the magnetic flux generated by the permanent magnet 5 and the magnetic flux generated by the coil 15 negate each other. As a result, the force attracting the armature 11 is decreased.
Accordingly, the distortion energy accumulated in the leaf spring 8 is released and the leaf spring 8 reverts to an original position. The printing wire 12 fixed to the tip of the armature 11 protrudes through the wire guide 13 to press an ink ribbon and a printing medium (neither are shown) against a platen (not shown). As a result, characters and graphic patterns can be printed out.
However, in a wire dot printing head having the above-described construction, the magnetic flux of the electromagnet which is used to negate the magnetic flux generated by the permanent magnet also leaks through the adjacent armatures and cores. Accordingly, magnetic interference causes a change to the magnetic flux of the core.
Further, the greater the number of the dot wires printing at the same time, the larger the change in the magnetic flux caused by the above-described magnetic interference. A larger exciting current is required to release an armature than if the respective dot wire were driven in isolation. Accordingly, the temperature of the printing head increases accompanied by an increase in the power consumption.
Further, if the printing head is to be miniaturized for use in a small printer, a flexible design is difficult to achieve due to the limited coil space and restrictions on the coil winding turns and diameters.